Steam generating unit



Jan. 14, 1964 M. H. KUHNER 3,117,560

STEAM GENERATING UNIT Filed Jan. 10, 1962 2 Sheets-Sheet 1 INVENTOR. maxH. Kuhner H 'rney Jan. 14, 1964 M. H. KUHNER 3,117,560

STEAM GENERATING UNIT Filed Jan. 10, 1962 2 Sheets-Sheet 2 JNV ENTOR.

max H. Kuhnev WWW ii ior% ey United States Patent Q STEAM GENERATINGUNIT Max H. Kuhner, Oahham, Mass., assiguor to Riley Stoker Corporation,Worcester, Mass, a corporation of Massachusetts Filed Jan. 10, 1962,Ser. No. 166,984 2 Claims. (Cl. 122478) This invention relates generallyto a steam generating unit and more particularly to apparatus arrangedto produce steam by burning fuel in a combustion chamber and bringingthe resultant gases into heat exchange relationship with fluidandvapor-containing tubes. This is a continuation-in-part of patentapplication Serial Number 781,662, filed December 19, 1958, nowabandoned.

It is customary in the art of steam generation to design each individualunit on a custom basis; that is to say, since every particularinstallation involves factors such as capacity, pressure, temperature,fuel and the like which differ from every other unit previously producedby a given manufacturer, it is necessary to design a unit which isparticularly adapted to the circumstances of that unit. The design andconstruction of steam generating units in this manner is very expensivebecause of the fact that each step of the design and manufacturingoperation is unique; in the manufacturing process, for instance, it isimpossible to make use of standard units in the manner common in otherindustries to reduce the unit cost. These and other difficultiesexperienced with the prior art devices of this type have been obviatedin a novel manner by the present invention.

It is, therefore, an outstanding object of the present invention toprovide a steam generating unit of such a design that, irrespective ofthe particular fuel used, the capacity of the unit may be increased inthe design stage merely by increasing the width by a proportionateamount.

It is another object of this invention to provide a steam generatingunit having unusual superheat and reheat temperature controlcharacteristics and in which the efficiency produce a substantially flatsuperheat-load characteristic curve without danger of tube fouling andslagging.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims, the following description andthe annexed drawings setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however,

of but a few of the various ways in which the principles of theinvention may be employed.

In said annexed drawings:

FIG. 1 is a vertical sectional view of a steam generating unit embodyingthe principles of the present invention, and

FIG. 2 is a vertical sectional View of a steam-and-Water drum associatedwith the unit shown in FIG. 1.

Referring first to FIG. 1, wherein are best shown the general featuresof the invention, the steam generating unit, indicated generally by thereference numeral 11 is shown as comprising a furnace 11 and a boiler12, the furnace producing in the usual manner hot products of combustionwhich pass in heat exchange relationship to the boiler. The furnace 11is provided with a front wall 13, a rear wall 14, and side walls 15,defining a combustion chamber 16. A bottom 17 is formed at the lowerportion of the furnace underlying the combustion chamher 16 and is ofthe water-cooled type consisting of a horizontal bank of water-filledtubes covered with a high temperature refractory material. A slagopening 18 is formed in the central portion of the bottom 17 and theedge of the opening is provided with a slag darn 19 of the usual type. Ashort distance above the bottom 17 the front and rear walls 13 and 14are provided with inwardly-diercted arches or abutments 21 and 22,respectively. Lying between them is a restricted passage .23. Theabutments 21 and 22 and the bottom 17 serve to define a restrictedhigh-temperature chamber 24. Mounted on the lower sides of the abutments21 and 22 are directional-flame inter-tube burners 25 and 26 which areof the type shown and described in thepatentof Craig, No. 2,759,460.Wind boxes 27 and 28 extend around the outer portions of the burners 25and 26, respectively, and the wind boxes are connected toa duct 2?surrounding the furnace just above the level of the abutments 21 and 22.Solid fuel enters the system from a bunker 31 and passes through afeeder 3:2 to a ball mill pulverizer 33; puverized coal then passesthrough ducts 34 and 35 to the burners 25 and 26, respectively. It willbe understood, of course, that, although pulverized coal is shown asbeing the fuel in the preferred embodiment, the principles of theinvention are as readily applicable to the burning of gas, oil or othersimilar fuels.

At the upper part of the combustion chamber 16 the front Walls 14 areprovided with arches or abutments 36 and 37 respectively, which define arestricted passage 38 lying therebetween. It will be understood that theabutments 21, 22, 36 and 37 are formed by bends in tubes which cover thesurfaces of the walls 13 and 14, as well as the side Walls 15. Somedistance above the abutments 36 and 37 the furnace is provided with aroof 39. The roof, the walls 13, 14 and 15 and the abutments 36 and 37define a superheater pass 41 having an outlet 42 at its upper end in thewall 13. A duct 43 extends from the outlet 42 to a regenerative airheater 44. The section of the duct between the outlet and the air heateris provided with a venturi 45 for the measurement of gas flow and with ahopper 46 located at the outer side of a bend in the duct, which hopperis adapted to receive dust and cinders. The other side of the air heateris connected to a stack 37 of the usual type. Incoming air is suppliedto the duct 29 by means of a fan 48 whose outlet is connected to the airheater 44 and the fan is connected, in turn, to a duct 49 leading to theair duct 29 carrying air to the burners 25 and 26.

The boiler 12 is provided with a steam-and-water drum 51 which isconnected bydowncomer tubes 52-to headers 53 and 54 underlying the floor17 of the furnace. Tubes extend from the headers 52 and 54 upwardlyabove the surfaces of the furnace, forming so-called water walls,

and, as has been stated, are bent outwardly to define the abutments 21,22, 36 and 37. A large number of the tubes lying along the front wall 13leave the water wall at the innermost point of the abutment 36 andextend vertically upwardly to form a supporting wall 55; in the sameway, some of the tubes lining the rear wall "14 leave the abutment 37 atits innermost position and extend vertically upwardly to form asupporting tube wall 56; the tubes in the walls 55 and 56 aresufficiently widely spaced to permit flow of gas therethrough. The tubewall 35 and the front wall 13 define a front pass 57 and a similar rearpass 58 is defined by the tube wall 56 and the rear wall 13 of thefurnace, while the remaining area between the tube wall 55 and the tubewall 56 is a central pass 56. The tubes lining the rear wall 14 of thefurnace, including the tubes in the tube wall 56, join thesteam-and-water drum at its rear portion, while the tubes lining thefront wall 13, including those which form the tube wall 55, join thesteam-andwater drum 51 at its forward portion. The tubes lining the sidewall 15 also are joined to the steam-and-water drum at its frontportion.

The upper part of the steam-and-Water drum 51 is connected to a smallsteam header 61 which, in turn, is connected to the lower ends of setsof superheater platens 62 and 63, associated with the front wall 13 andthe rear wall 14, respectively. Each of the platens 62 consists of anumber of tubes in substantially tangential relationship originating attheir lower ends in a short vertical header 64 and terminating at theirupper ends in a horizontal outlet header 65 which resides within theabutment 36. Similarly, each of the platens 63 associated with the rearwall 14 originates in a short vertical header 66 located outside therear wall 14 some distance above the abutment 22 and terminates in ashort horizontal outlet header 67 located within the abutment 37. Itwill be understood that there are a substantial number of platens 62associated with the front wall 13 and a similar number of platens 63associated with the rear wall 15, these being evenly spaced across thewidth of the combustion chamber 16. It should be noted that theinnermost edges of the platens 62 and 63 do not extend further into thecombustion chamber than the abutments 21, 22, 36 and 37 and, thus, in asense, they reside within pockets formed at the front and rear of thefurnace. The outlet headers 65 and 67 associated with the platens 62 and63 are joined together and to the in-put side of a spray-typedesuperheater 68, the outlet of which is connected to the lower end of asecondary superheater section 69 which resides partly in the center pass59 and partly in the front pass 57, the upper end being connected to asuperheated steam header 71 which, in turn, is connected to thehigh-pressure section of a steam turbine, not shown. A return line 72from the turbine is connected to the upper end of a primary reheatersection 73 located partly in the center pass 59 and partly in the rearpass 58. The lower or output end of the reheater section 73 is connectedto the lower end of a secondary reheater section 74 which is located inthe rear pass 58 only. The upper end of the reheater section 74 isconnected to a reheated steam header 75 which, in turn, is connected tothe low-pressure section of the steam turbine.

Referring to FIG. 2 which shows the details of the steam-and-water drum51, it can be seen that the downcomer tubes 52 are connected into thedrum, as are the steaming tubes 76 originating in the rear wall 14, andthe steaming tubes 77 originating in the front wall 13 and the sidewalls 15. The drawings also show the manner in which the tubes 78 leavethe drum to connect to the header 61. It will be understood that theelongated drum is at least as long as the furnace is wide. It will alsobe understood that the tubes 76 and 77 and 78 are located in theportions of the drum between the ends. A bafiie plate 79 extendslengthwise f the drum in the lower portion thereof and forms acompletely sealed receptacle which is open upwardly and is connected tothe downcomer tubes by means of fittings 81. On the side of the drumadjacent the tubes 77 the bafile plate 79 is formed to provide avertical portion at the upper edge of which is mounted a bracket 82supporting the lower end of a series of bafile plates 83 which aresuitably fastened to the drum at the upper end; a similar bracket 84extending from the opposite upper edge of the bafl'le plate 79 supportsthe lower end of similar bathe plates 85 adjacent the opening of thetubes 76 into the drum. Running centrally down the middle of the drum isa feed water pipe 86 which is connected to the input leg of U-shapedcondenser members 87 and 88 located immediately inwardly of the bafileplates 83 and 85, respectively. The condenser elements 87 and 88 are ofthe type shown in the patent to Andrews No. 2,424,212, issued July 22,1947. The other leg of the condenser 87 is provided with adownwardly-turned 4 fitting 89 and the condenser $3 is provided with asimilar fitting 91, the lower ends of the fittings residing well withinthe trough formed by the baflie plate 79. The condensers are, of course,covered with fins to promote heat exchange between the steam passingthereover and the feed water running on the inside of the tubes.

Underlying the entrance to the tubes 78 in the upper central portion ofthe drum is a trough 92 suitably supported and having fastened to itsupper lateral edges drier cartons 93 and 94 whose upper edges aresuitably fastened and sealed against the surface of the drum on theopposite sides of the entrances to the tubes 78. From the bottom of thetrough 92 extend drain pipes 95 and 96 on opposite sides of the feedwater pipe 86, the ends of the drain pipes residing well within thedown-comer tubes 52 and being slightly flared to increase the ejectoraction of the fluid flowing down the down-comer tube to create awithdrawal of fiuid from the pipes 95 and 96 and the trough 92.

The operation of the apparatus will now be readily understood in view ofthe above description. Fuel enters the furnace in the usual manner,after suitable pulverization, through the burners 25 and 26 and is mixedwith the air originating in the windboxes 27 and 28. Combustion takesplace in the high-temperature chamber 24 underlying the abutments 21 and22 and the hot products of combustion pass upwardly through the furance.The combustion gas how is distributed between the front pass 57, therear pass 58, and the central pass 59 by means of dampers 97, 98 and 99residing in the outlet 42 of the superheater pass 41. A suitable bafile101 serves further to separate the pass 57 from the remainder of theupper part of the furnace. The gas passes through the duct 43, throughthe venturi 45 where the amount of flow is measured, and through the airheater 44 to the stack 47. In the hopper 46 a certain amount of theunburned particles are deposited, and in the air heater 44 the gas, ofcourse, comes into heat exchange relationship with the incoming airwhich originates in the fan 48 passing through a duct 49 to the burners.Feed water entering through the feed water pipe 36 passes through thecondensers 87 and 88 and is introduced into the trough formed by thebaffle plate 79; it then flows downwardly through the downcomer tubes 52to the headers 53 and 54. The water is distributed by these headers intothe water-wall tubes lining the walls 13, 14, and 15 of the furnace andthey flow upwardly in these walls. At some point during the upwardjourney through the tubes, the water is converted to steam; the steameventually leaves the tubes 76 and 77 and enters the steam-and-waterdrum. The steam originating in the tubes 77 passes over the bafiieplates 83, thus removing the larger particles of water. The steam thenpasses through the condenser 87 in which a certain amount of steam iscondensed, thus wetting the surface of the condenser and providing thesteam with a purifying action. The steam then passes over the upper lipof the trough 92, through the drier carton 93 and out of the drumthrough the tube 78 to the header 61; the same action takes place in thesteam emerging from the tube 76. This steam passes through the baffleplates 85, over the condenser 88, and through the drier carton 94 beforeentering the tubes 98. The steam which arrives at the header 61eventually passes downwardly to the header 66 and into the tubes formingthe platens 62 and 63. While the steam is in the platens it receivesheat mainly by radiation from the gases passing upwardly through thecombustion chamber 16. A certain amount of convection heat exchangetakes place, of course, but because of the fact that platens do notextend out beyond the abutments, the heat received is to a great extentthat of radiation. In any case, all steam passing through the platens 62and 63 eventually passes through the desuperheater 68 where thetemperature is reduced by a water spray in accordance with a measurementof final steam temperature aided by controls which do not form a part ofthis invention and, therefore, are not shown. The superheated steamleaves the desuperheater 68, enters the lower end of the secondarysuperheater 69, and passes through the sinuous coils thereof to emergeinto the superheated steam header 71 on its way to the turbine. Afterpassing through the turbine and having a considerable portion of itsthermal energy converted into mechanical power, the steam returns in theline 72 and passes downwardly to the primary reheater 73, eventuallypassing into the secondary reheater 74 in which the flow is in the samedirection as gas flow. It will be noted that the heat exchangerelationship between the gas and the two reheater sections isconductive. Manipulation of the dampers 97, 98 and 99 proportions thegas in its flow through the passes 57, 5S and 59.

It should be noted that an econornizer section 103 extends across thetop of passes 57, 58 and 59 While an economizer section 102 lies in thefront pass 57 only. The feed water passes through the economizer sectionN2 in the same direction as the gas fiow while it passes through theeconomizer section 103 in counter-flow, the upper end of the section 162being connected to the feed water pipe 86 while the upper end of theeconomizer section 103 is connected to the feed water pump in the usualmanner.

Because the inter-tube, directional-flame burners of the furnace are alllocated at one elevation in two opposed walls of the furnace and becausethe types of fuel to be burned require no change in burner location, itis possible to adapt as standard an ideal furnace configuration. Thedimensions of the two opposed burner walls and the height of the furnacemay, thus, remain the same for all capacities for which this design isused. Only the distance between the side walls and the furnace widthneed be changed to adopt the design for larger or smaller steamgenerating capacity. The changein furnace width is proportional to thenumber of burners required for the quantity of fuel to be burned.

Because combustion is completed in this furnace substantially within thehigh temperature chamber 24, the use of radiant superheater platens ispractical and safe. When radiant steam-superheating surfaces are usedalong the walls of the conventional front-fired or tangentiallyfiredfurnaces, flame control is very critical; in these cases flameimpingement on steam heating surfaces must be avoided top revent damageof the radiant tubes. The present invention permits the use of radiantsuperheater platens placed over the upper section of the burner walls.It would be the usual practice to place the radiant superheater platenson 24" to 30" centers. They are not subject to direct flame impingementbut receive radiant heat from the high temperature chamber of thefurnace. They also receive a certain amount of convection heat from thecombustion gas passing vertically upwardly from the furnace and, thus,flowing parallel with the vertical superheater tube platens. The heatingsurface of the radiant superheater platens can be so proportioned to theconvection superheater surface that an almost constant final temperaturecan be produced over a very wide range of steam load. The droopingeffect of heat absorption by radiation with increasing steam flow isideally balanced with the rising characteristic of convection heatabsorption in the convection superheater. The spray-type desuperheaterplaced between the radiant and the convection sections of thesuperheater need be of relatively small capacity to maintain thespecified final steam temperature over a wide range of load. Note thatthe saturated steam enters the low end of the radiant superheaterplatens where combustion gas temperature is highest and where cooling ofthe platen tubes is most important. Note also that the entiresuperheater system is completely drainable. This permits rapid start-upof the boiler because time need not be wasted in evaporating thecondensate from the usual hanging superheater loops to open thesuperheated tubes to steam passage.

The reheater is of the convection type and is also fully drainable.Reheat steam temperature is controlled by the use of gas flowproportioning dampers located beyond the econ om-izer where the averageflue gas temperature is below 800 F. Combustion gas flow is divided sothat the quantity required to produce the specified final reheattemperature must pass over the reheat surface while the rest of the gasflows over the economizer surface. Sub stantially all of the combustiongas passes over the convection superheater regardless of the position ofthe gas proportioning dampers. While the boiler is being brought up topressure and until steam flow through the re'heater is established, therehcater gas pms is completely closed so that the reheater surface iswell protected against excessive temperatures until the turbine is inoperation and reheat steam flow is established. This feature againpermits rapid build-up of pressure because the firing rate need not begradually raised for the protection of the uncooled reheater surface.

By placing the superheater and economizer surfaces above the furnace,the stack effect of the vertically rising combustion gas assists inovercoming friction loss so that the draft loss through the boiler islower than with the conventional front-fired design where convectionsurfaces are placed in a second down-pass. The overall height of thestructure is, however, not much greater than for the Well-knownfront-fired arrangement. Liberal height is desirable for high pressurenatural circulation boilers to insure good-circulation by having highcolumns of solid water in the large feeder pipes supplying the water tothe steaming section of the furnace. Withfurnace designs of the typeshown, heat is released within the lowest section of the furnace;circulation in the furnace wall tubes is, thus, much more secure thanwith horizontally-fired furnaces where the burners are placed aconsiderable distance above the furnace fioor.

The present invention inconpor-ates the feature of having thesteam-carrying tubes entering the steam release drum on both side of thevertical center line. The rate of steam release per linear foot of drumlength may thus be almost double thatof the earlier designs in whichsteam carrying tubes enter-the drum over only one side, usually thefront half of the drum shell. The compact design shown permits hangingor suspension of the entire boiler unit on comparatively simplestructural steel framing. The standardized cross-sectional elevation ofthe boiler should resolve in savings of manufacturing cost andengineering.

Other modes of applying the principles of the invention may be employed,change being made as regards the details described, provided thefeatures stated in the following claims or the equivalent of such beemployed.

The invention having been thus described, what is claimed as new anddesired to secure by Letters Patent is:

1. A steam generating unit, comprising (a') a vertically-elongatedfurnace,

(b) a coiler having a steam-and-water drum containing two sets ofsteam-cleaning apparatus one on either side of the vertical centerlineof the drum,

(0) steam purifier apparatus in the drum,

(0.) water-walls defining a combustion chamber,

(e) a slag basin at the lower portion of the combustion chamber havin acentrally-located tap hole,

(f) two of the water-walls being in opposed relation and being verticalthroughout most of their heights,

(g) a lower abutment extending from each of the said two opposedwater-walls a short distance above the slag basin, each abutmentextending horizontally completely across the width of its water-wall,the abutments extending toward one another on the same level to define arestricted passage therebetween,

(12) directional-lame inter-tube burners mounted on the undersides ofthe abutments and directed toward the slag basin, the abutments definingwith the slag basin a high-temperature cell in which combustion issubstantially completed during operation, the upper abutments extendingfrom each of the said two water-walls at the upper end of the combustionchamber, the upper abntrnents defining therebetween a restricted furnaceoutlet passage,

(i) a convection pass extending upwardly from the said furnace outletpassage, the steam-and-water drum being located above the superheaterpass,

(j) superheater platens constituting primary superheate-r sectionsextending inwardly from the said opposed water-walls and extendingvertically for a substantial portion of the distance from the lower tothe upper abutment of a given Water-wall,

(k) a substantial number of water-wall tubes leaving each upper abutmentand extending upwardly through the convection pass to define it into afront portion, a rear portion, and a central portion,

(1) a secondary superheater section extending across the lower part .ofthe front and central portions and arranged to carry steam therethroughin the direc tion of gas flow,

(m) a primary reheater section located in the upper parts of the rearand central portions for fiow of steam therethrough in counter-fiow,

(u) a secondary reheater section located in the lower part of the rearportion for flow of steam therethrough in the direction of gas flow,

(0) an economizer having a high-temperature section located in the upperpart of the front pass for how of feed water in the direction of gasflow and having a low-temperature section extending across the uppermostpart of the front, central and rear portions for flow of feed water incounterflow,

(p) and dampers located in the upper part of the said portions tocontrol the flow of gas therethrough, the tubes originating in one ofthe said opposed Waterwalls terminating at one side of the said drum tosupply steam to one set of steam-cleaning apparatus and the tubes tt'romthe other of said opposed waterwalls terminating at the other side ofthe drum to supply steam to the other set of steam-cleaning apparatus.

2. A steam generating unit, comprising (a) a vertically-elongatedfurnace,

(b) a boiler having a steam-and-water drum containing two sets ofsteam-cleaning apparatus one on either side of the vertical centerlineof the drum,

(0) steam purifier apparatus in the drum,

(d) water-walls defining a combustion chamber,

(e) a slag basin at the lower portion of the combustion chamber having acentrally-located tap hole,

(7) two of the water-walls being in opposed relation and being verticalthroughout most of their heights,

(g) a lower abutment extending from each of the said two opposedwater-walls a short distance above the slag basin, each abutmentextending horizontally completely across the width of its water-wall,the abutments extending toward one another on the same level to define arestricted passage therebetween,

([1) directionahfianie inter-tube burners mounted on the undersides ofthe abutments and directed toward the slag basin, the abutments definingwith the slag basin a high-temperature cell in which combustion issubstantially completed during operation, the upper abutments extendingfrom each of the said two Water-walls at the upper end of the combustionchamber, the upper abutments defining therebet-ween a restricted furnaceoutlet passage,

(1') a convection pass extending upwardly from the said furnace outletpassage, the steam-and-water drum being located above the superheaterpass,

(j) a substantial number of water-wall tubes leaving each upper abutmentand extending upwardly through through the convection pass to define itinto a front portion, a rear portion, and a central portion,

(k) a superheater section extending across the convection pass andarranged to carry steam therethrough in the direction of gas flow,

(I) a reheater section located in the convection pass for flow of steamtherethrough in counter-flow,

(m) an economizer having a section located in the convection pass forflow of feed water in the direction of gas flow,

-(n) and dampers located in the upper part of the said portions, tocontrol the flow of gas therethrough, the tubes originating in one ofthe said opposed waterwalls terminating atone side of the said drum tosupply steam to one set of steam-cleaning apparatus and the tubes fromthe other of said opposed waterwalls terminating at the other side ofthe drum to supply steam to the other set of steam-cleaning apparatus.

References Cited in the file of this patent UNITED STATES PATENTS2,628,598 Van Brunt Feb. 17, 1953 2,715,451 Raynor Aug. 16, 19552,815,007 Sprague et al. Dec. 3, 1957 2,821,175 Seidl Jan. 28, 1958

1. A STEAM GENERATING UNIT, COMPRISING (A) A VERTICALLY-ELONGATEDFURNACE, (B) A COILER HAVING A STEAM-AND-WATER DRUM CONTAINING TWO SETSOF STEAM-CLEANING APPARATUS ONE ON EITHER SIDE OF THE VERTICALCENTERLINE OF THE DRUM, (C) STEAM PURIFIER APPARATUS IN THE DRUM, (D)WATER-WALLS DEFINING A COMBUSTION CHAMBER, (E) A SLAG BASIN AT THE LOWERPORTION OF THE COMBUSTION CHAMBER HAVING A CENTRALLY-LOCATED TAP HOLE,(F) TWO OF THE WATER-WALLS BEING IN OPPOSED RELATION AND BEING VERTICALTHROUGHOUT MOST OF THEIR HEIGHTS, (G) A LOWER ABUTMENT EXTENDING FROMEACH OF THE SAID TWO OPPOSED WATER-WALLS A SHORT DISTANCE ABOVE THE SLAGBASIN, EACH ABUTMENT EXTENDING HORIZONTALLY COMPLETELY ACROSS THE WIDTHOF ITS WATER-WALL, THE ABUTMENTS EXTENDING TOWARD ONE ANOTHER ON THESAME LEVEL TO DEFINE A RESTRICTED PASSAGE THEREBETWEEN, (H)DIRECTIONAL-FLAME INTER-TUBE BURNERS MOUNTED ON THE UNDERSIDES OF THEABUTMENTS AND DIRECTED TOWARD THE SLAG BASIN, THE ABUTMENTS DEFININGWITH THE SLAG BASIN A HIGH-TEMPERATURE CELL IN WHICH COMBUSTION ISSUBSTANTIALLY COMPLETED DURING OPERATION, THE UPPER ABUTMENTS EXTENDINGFROM EACH OF THE SAID TWO WATER-WALLS AT THE UPPER END OF THE COMBUSTIONCHAMBER, THE UPPER ABUTMENTS DEFINING THEREBETWEEN A RESTRICTED FURNACEOUTLET PASSAGE, (I) A CONVECTION PASS EXTENDING UPWARDLY FROM THE SAIDFURNACE OUTLET PASSAGE, THE STEAM-AND-WATER DRUM BEING LOCATED ABOVE THESUPERHEATER PASS,